Scientists have discovered the world’s largest known bacterium, which appears as white filaments the size of human eyelashes, in a marsh in Guadeloupe.
About 1 cm long, the strange organism, Thiomargarita magnifica, is about 50 times larger than all other known giant bacteria and the first to be visible to the naked eye. The thin white strands were discovered on the surface of decaying mangrove leaves in shallow tropical marine marshes.
The finding came as a surprise because, according to models of cellular metabolism, bacteria just shouldn’t get that big. Previously, scientists had suggested a possible upper size limit about 100 times smaller than the new species.
“To put it into context, it would be like a human meeting another human as big as Mount Everest,” said Lawrence Berkeley National Laboratory scientist Jean-Marie Volland, who co-authored the study.
The organism was discovered by Olivier Gros, professor of marine biology at the University of the West Indies in Guadeloupe, while looking for symbiotic bacteria in the mangrove ecosystem.
“When I saw them, I thought: strange,” Gros said. The lab first performed microscopic analyzes to establish that the strands were single cells. Closer inspection also revealed a strange internal structure. In most bacteria, DNA floats freely inside the cell. Thiomargarita magnifica seems to keep its DNA more organized inside membrane-bound compartments throughout the cell. “And that’s very unexpected for a bacterium,” Volland said.
The bacterium was also found to contain three times as many genes as most bacteria, and hundreds of thousands of copies of the genome spread throughout each cell, making it unusually complex.
Scientists don’t yet know how the bacteria evolved to become so large. One possibility is that it adapted to escape predation. “If you grow hundreds or thousands of times bigger than your predator, you can’t be consumed by your predator,” Volland said.
However, getting big would have meant losing some of the bacteria’s traditional advantages, including their unique ability to move around and colonize new niches. “By leaving the microscopic world, these bacteria have definitely changed the way they interact with their environment,” Volland said.
The bacterium has yet to be found in other locations – and had disappeared from the original site when researchers returned recently, possibly because they are seasonal organisms. But in the paper, published in the journal Science, the authors conclude that the discovery “suggests that larger, more complex bacteria may be hiding in plain sight”.